The present invention relates to a microporous polyolefin composite membrane for use as a separator of a nonaqueous battery such as a lithium battery, and more specifically, a microporous polyolefin composite membrane which has an excellent permeability and mechanical strength as well as a high safety to shut down at a low temperature the permeability when an unusually large amount of heat is generated due to short circuit of the battery.
Microporous polyolefin membranes are widely used in various applications such as various filters, battery separators, electrolytic capacitor separators, etc. Since a lithium battery utilizes lithium metal and lithium ion, an aprotic organic polar solvent is used as the solvent for electrolyte such as a lithium salt. Therefore, as the separator being disposed between a negative electrode and a positive electrode, a microporous membrane or a nonwoven fabric each made of a polyolefin such as polyethylene, polypropylene, etc. is used because they are insoluble in the aprotic organic polar solvent and chemically stable against the electrolyte and electrode active materials.
The lithium secondary battery is known as one of the secondary batteries of the highest energy density, because it has an electromotive force as high as 2.5 to 4 V and utilizes lithium having a low atomic weight as the main electrode active material. A button type or UM-3 type lithium secondary battery of a small capacity has been used as a backup battery for computer memory, a power source for handy phones, etc. However, since a capacity of about 10 kWh is required when a battery is applied to electric cars, etc., it has been demanded to develop a lithium battery having an enhanced capacity and an enhanced output power. Since the aprotic organic polar solvent has a low electroconductivity, the electric current density of the lithium battery is low. Therefore, a large surface area of the electrodes is necessary to obtain a large-size lithium battery enhanced in both the capacity and output power.
The separator for use in such a battery is desired to have suitably small pores in its matrix. However, since the contact surface between the electrodes and the separator reduces the effective surface area of the electrodes, the separator is preferable to have a rough surface on the microscopic scale and a relatively large pore size on its surface. Also, the separator should have a reasonable thickness which keeps both the electrodes spaced each other with an sufficient distance for safety's sake. Further, the holding capacity of the separator for the electrolyte solution should be enhanced to assure good battery characteristics such as discharge property and cycle property.
Recently, there has been proposed various methods for producing a high-strength and high-modulus microporous membrane from a ultrahigh molecular weight (UHMW) polyolefin. For example, in one of the methods, a UHMW polyolefin composition is heated to an elevated temperature in a solvent to form a solution of UHMW polyolefin. This solution is extruded to form a gel sheet which is then stretched under heating and extracted to remove the solvent from the stretched sheet to form a microporous structure in the sheet (Japanese Patent Laid-Open Nos. 60-242035, 61-195132, 61-195133 and, 63-39602, U.S. Pat. No. 4,873,034, etc.). In another method of producing a microporous polyolefin membrane from a highly concentrated solution of UHMW polyolefin, the molecular weight distribution of a polyolefin composition containing UHMW is controlled within a specific range (U.S. Pat. No. 5,051,183). Although the microporous polyolefin membrane known in the art has a small pore size, it does not meet the requirement of the above secondary battery to have a high holding capacity and a high output level.
The lithium battery generates an intense heat due to short-circuit of the electrodes to cause ignition of lithium. Therefore, the battery separator should have a function to shut down the electric current flow through the separator membrane by clogging the pores, before lithium catches fire, with molten material which constitutes the membrane. However, since the known microporous polyolefin membranes are molten at a higher temperature, the pores are not effectively clogged at a temperature sufficiently low to prevent the ignition of lithium. Considering the above problems, it is desirable to lower the shutdown temperature to assure the safety of using the lithium battery. Thus, the lower the shutdown temperature is and the larger the temperature difference between the shutdown temperature and the breakdown temperature of the membrane is, the more excellent the battery is in its safety and reliability.
Japanese Patent Application No. 60-23954 discloses a technique to provide a battery separator with the shutdown function at short circuit. This document teaches that the use of a single-layered microporous film of polypropylene or polyethylene is desirable to prevent the ignition or explosion of the battery due to the meltdown of the separator material being caused when the temperature in the battery is elevated by Joule's heat generated in the external short circuit. When a battery is externally short-circuited, the temperature of the battery rises by Joule's heat to reach a melting point of the separator material, at which the single-layered microporous film of polypropylene or polyethylene begins to melt. The melt of separator material clogs the pores in the separator to prevent the ions from being transported through the separator, and makes the separator electrically insulating to shut off the current flow. Therefore, the temperature rise stops, and the ignition or explosion of the battery can be prevented.
However, at an elevated temperature around the melting point of a thermoplastic resin such as polyolefin, the cohesive force in the separator is so reduced that a separator comprising a single-layered microporous membrane is likely to be broken. Therefore, the problem of the ignition or explosion of battery still remains unsolved.